State of the art electronic instruments, such as sampling oscilloscopes, cannot measure electrical signals with a sub-picosecond time resolution. Such measurements are required in characterizing today's state of the art electronic devices that operate in the commensurate time regime. Measurements can be made, however, with electrooptic instruments based on electrooptic sampling techniques.
Generally, electrooptic sampling techniques utilize ultrashort optical pulses in combination with an electrooptic phenomenon, such as photoconductivity or photoemission, to make jitter-free measurements of electrical signals which are present at internal nodes of electronic devices. Two main electrooptic sampling techniques which have been developed are based on either the Pockels effect or the photoconductive effect.
In electrooptic sampling based on the Pockels effect, an electrical signal propagating along a pair of conductive lines is measured by directing an optical beam through an electrooptic crystal (a crystal having a high electrooptic coefficient) that is placed adjacent to the pair of lines. A field-induced birefringence in the crystal, resulting from the electrical signal, changes the polarization of the optical beam as it propagates through the crystal. Cross polarizers in combination with photodetectors detect this change in polarization and, thus, measure electrical signal activity on the conductive pair of lines. See, for example, J. A. Valdmanis et al., IEEE Journal of Quantum Electronics, Vol. QE-22, No. 1, pp. 69-78 (1986); U.S. Pat. No. 4,446,425; and U.S. Pat. No. 4,618,819.
In contrast, electrooptic sampling based on the photoconductive effect utilizes two optical beams, a pump beam and a probe beam, to generate and to detect an electrical signal, respectively. The pump beam illuminates a first gap comprising photoconductive material between a biased transmission line and a main transmission line. Free carriers produced upon illuminating the first gap reduce the resistance of the material so that charge is transferred from the biased transmission line onto the main transmission line. This charge transfer results in an electrical signal being injected onto the main transmission line. The probe beam illuminates a second gap comprising photoconductive material at a delay time, t+.tau., between the main transmission line and a sampling electrode. This, in turn, diverts a portion of the charge on the main transmission line onto the sampling electrode. By integrating and averaging the diverted charge on the sampling electrode line as a function of .tau., the injected electrical signal can be measured without physically contacting the transmission lines. Furthermore, the injected electrical signal can be directed into an electronic device and the electronic response measured by the probe beam in a similar fashion as above. See, for example, D. H. Auston, Appl. Phys. Lett., 37 (4) pp. 371-373 (1980); and U.S. Pat. No. 4,482,863.
Both electrooptic sampling techniques described above have become viable means for the measurement of ultrashort electrical signals. Although electrooptic sampling based on the Pockels effect has a higher temporal resolution (300 fsec) than photoconductive techniques (2 psec), the former has a lower voltage sensitivity and, in practice, requires an averaging of a high number of measurements for an acceptable signal to noise ratio to be achieved.